The E. coli DNA polymerase I is a DNA-dependent DNA polymerase that possesses both 3' -> 5' and 5' -> 3' exonuclease activities. It is a single-chain protein with a mass of about 109,000 Da that requires magnesium as a cofactor. Each of its three enzymatic activities are encapsulated into distinct domains of the holoenzyme, such that proteolytic deletions can be generated that lack one or more of the activities. The so-called Klenow fragment is one such molecule that is widely used in recombinant DNA work.
DNA polymerase I was used frequently in the early days of recombinant DNA technology for radiolabeling DNA and synthesizing cDNA. However, other enzymes have proven to be more effective for these purposes, including a proteolytic fragment of DNA polymerase I called Klenow fragment and T4 DNA polymerase. The holoenzyme DNA polymerase I is no longer frequently used.